Unlike most of the other characters, Evelyn Marsh (Annette McCarthy) did not actually live in Twin Peaks, but one of the towns nearby, which James Hurley "drifts" to, after he flees Twin Peaks in the wake of Maddy Ferguson's murder. She is very wealthy, and she and her absentee husband, Geoffrey/Jeffrey, own a small fleet of expensive cars, which she initially drafts James in to repair.
Evelyn is revealed to be a complex and unpredictable character. She attracts James through a combination of her physical attributes and the pity he feels for her, when she reveals that Geoffrey frequently beats her. In reality, she is involved with a third man, Malcolm Sloan, who lives with the family under the guise of being her brother and acting as chauffeur, and the two have hatched a plan for killing Geoffrey. She engages James in a kind of boy toy sexual relationship, intending to set him up as a fall guy for her husband's death in a car accident. When Donna Hayward comes to find James and talk with him, Evelyn taunts Donna with her relationship with James and warns her off.
Following her husband's murder, Evelyn appears to soften towards the honest, naive James and show repentance for her actions, giving James enough warning to get out of the house before the arrival of the police. She gives his full name to the police and even indicates where they might find him, but seems to waver. When he comes back to talk to her, she pleads for him to leave. Malcolm knocks James out and tries to persuade Evelyn to shoot him, concocting a story to cover the situation, but in the end Evelyn can't go through with it and she shoots Malcolm dead instead when he tries to attack her, allowing James to go free. After this, all we hear of Evelyn is that James is to give evidence at her trial.